s390/vmemmap: remember unused sub-pmd ranges

With a memmap size of 56 bytes or 72 bytes per page, the memmap for a
256 MB section won't span full PMDs. As we populate single sections and
depopulate single sections, the depopulation step would not be able to
free all vmemmap pmds anymore.

Do it similarly to x86, marking the unused memmap ranges in a special way
(pad it with 0xFD).

This allows us to add/remove sections, cleaning up all allocated
vmemmap pages even if the memmap size is not multiple of 16 bytes per page.

A 56 byte memmap can, for example, be created with !CONFIG_MEMCG and
!CONFIG_SLUB.

Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200722094558.9828-9-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>